Method and apparatus for total hemoglobin measurement

ABSTRACT

The present invention pertains to a method and apparatus for total hemoglobin measurement. A modulated optical signal based on a digital code sequence is transmitted to human tissue. A temporal transfer characteristic is determined from the modulated optical signal. Total hemoglobin is determined based on the temporal transfer characteristic.

RELATED APPLICATIONS

The present application claims the benefit of and is a continuation ofU.S. patent application Ser. No. 12/875,983, filed on Sep. 3, 2010,entitled “Method and Apparatus for Total Hemoglobin Measurement,” whichis continuation-in-part of U.S. patent application Ser. No. 11/381,443,filed on May 3, 2006, entitled “Method and Apparatus for Lymph NodeMapping,” all of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The field of the present invention pertains generally to optical imagingusing near-infrared light, including more specifically, to the opticaldetection of sentinel lymph node location in order to guide surgicalprocedures.

BACKGROUND

Sentinel lymph node biopsy is a surgical procedure that involvesremoving a small sample of lymph tissue and examining it for signs ofcancer. As an alternative to conventional full lymph node dissection, itis increasingly used as the standard of care in the staging of breastcancer and melanoma. The sentinel lymph node (SLN) is the first node, orgroup of nodes, in the lymphatic network to come into contact withmetastatic cancer cells that have spread from the primary tumor site.SLN biopsy allows a physician to obtain information about the otherlymph nodes in the network without exposing the patient to the risks ofconventional surgery. Further surgery to remove other lymph nodes may beavoided if no cancer cells are found in the sentinel lymph nodes.

SLN biopsy usually begins with the injection of a radioactive tracer(technetium-99 sulfur colloid), a blue dye, or both into the area aroundthe original cancer site. Lymphatic vessels carry the tracer to thesentinel node (or nodes); this is the lymph node most likely to containcancer cells. Prior to surgery, a wide field-of-view gamma camera istypically used to image the location of the radiotracer. Images aregenerally taken from multiple positions and perspectives, resulting in amap of the drainage pattern of lymphatic fluid from the skin to thelymph nodes. By showing where the cancer is likely to have spread, themap enables the surgeon to plan the full procedure prior to the firstincision. During surgery, the surgeon achieves further guidance eitherthrough direct visualization of the injected blue dye or by detectingthe radioactive tracer with a hand-held gamma probe. After surgery, thelymph node is sent for pathological examination that can includemicroscopic inspection, tissue culture, or immunological tests.

The current approach of using radioisotopes for SLN mapping has severaldrawbacks. First, while the radiation risk to patients and medicalpractitioners is relatively low compared to other medical procedures,the handling of radioisotopes still requires special precautions.Second, the procedure requires the coordination of both surgical andnuclear medicine personnel, resulting in both scheduling issues andincreased cost. Lastly, the time required for the radiotracer to travelthrough the lymphatic system can be as long as several hours. It ishighly desirable to have an alternative system that could be usedwithout radiotracers and that a surgeon could utilize without theinvolvement of other specialists. It is also desirable to have a systemthat uses a contrast agent with more rapid kinetics.

Diffuse optical imaging techniques are known in medical and biologicalapplications. Overviews of diffuse optical imaging techniques can befound in “Recent Advances in Diffusion Optical Imaging” by Gibson, etal, Phys. Med. Biology, vol. 50 (2005), R1-R43 and in “Near-infraredDiffuse Optical Tomography,” by Hielscher, et al, Disease Markers, Vol.18 (2002), 313-337. Briefly, diffuse optical imaging involves the use ofnear-infrared light incident upon a sample of interest. An example inthe medical and biological field is optical mammography where nearinfrared light is used to illuminate breast tissue. A detector is placedon the opposite side of the breast from the incident light some distanceaway and collects scattered light from the breast tissue. The scatteredlight of interest that is detected may be directly scattered incidentlight or scattered fluorescence light caused by the excitation of aninjected fluorescing material that fluoresces when exposed to theincident light. By measuring the amplitude of the light of interest atthe detector and the distribution of photon arrival times at thedetector for various source and detector positions, a reconstruction ofthe underlying tissue optical properties can be made. An overview ofimage reconstruction techniques can be found in the citations given inthe aforementioned review articles.

Measurements of the photon flight-time distributions are typicallycarried out using either a time-domain or a frequency-domain technique.In the time-domain technique, the sample is excited with pulse of lightfrom a pulsed laser and the scattered light is measured using a detectorwith single-photon sensitivity. The detector measures the time delaybetween the excitation pulse and the first detected photon. Theflight-time distribution is determined by using many repeated pulses andbuilding up a histogram of the measured time delays. Unfortunately, thepulsed laser sources and single-photon detectors are relativelyexpensive. Because detection is typically done at the single-photonlevel, it can require a significant amount of time to build-up enoughdata to approximate the flight-time distribution. One disadvantage ofthe frequency-domain approach is that it is not a direct measurement ofthe photon flight time. Rather, it provides an estimate of the meanflight time based on the phase shift between a detected signal and theexcitation signal. In some cases, more accurate image reconstructionscan be obtained using more complete measurements of the flight-timedistributions. This data is not readily obtained with frequency-domaininstrumentation. A further disadvantage of the frequency-domain approachis the need for accurate high-frequency analog electronics. An overviewof both the time-domain and frequency-domain techniques can be found inthe above-referenced article by Hielscher, et al.

U.S. Pat. No. 5,565,982 discloses a time-resolved spectroscopy systemusing digital processing techniques and two low power, continuous wavelight sources. The disclosed system requires two light transmitters ofdifferent wavelengths modulated with separate codes for interrogating asample of interest. Properties of the sample are inferred bydifferential comparison of the return signals from each of the two lightsources. It is undesirable to have two distinct light sources due to thecost and complexity involved. Furthermore, the noise level associatedwith a measurement made with two separate light sources will be higherthan with a single source even if the codes used to drive the twosources are orthogonal. It is desirable to have a means of interrogatinga particular tissue volume with a single light source at one wavelengthin order to obtain temporal information.

What is needed is an imaging system capable of sentinel lymph nodemapping that does not require the use of radiotracers. Furthermore, thesystem should be implemented with low-power continuous-wave lightsources and digital electronics.

SUMMARY

The present invention pertains to a method and apparatus for totalhemoglobin measurement. The total hemoglobin measurement system has asignal generator for generating a digital modulation signal representinga code sequence and an optical illumination source for receiving thedigital modulation signal and for transmitting a modulated opticalsignal along an optical transmission path to human tissue in response tothe digital modulation signal. The total hemoglobin measurement systemalso has a detector for receiving the modulated optical signal aftertransmission through human tissue and a processor for determining atemporal transfer characteristic for the optical signal and fordetermining the oxygen level based on the temporal transfercharacteristic. In another embodiment, a method for measuring totalhemoglobin involves generating a digital modulation signal associatedwith a code sequence and generating a modulated optical signal based onthe digital modulation signal. It also involves transmitting themodulated optical signal to human tissue and receiving a modifiedversion of the modulated optical signal after transmission through humantissue. It also involves determining a temporal transfer characteristicfor the modified version of the modulated optical signal and determiningtotal hemoglobin based on the temporal transfer characteristic.

These and other objects and advantages of the various embodiments of thepresent invention will be recognized by those of ordinary skill in theart after reading the following detailed description of the embodimentsthat are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements.

FIG. 1 is a functional block diagram of the major components of apreferred photon measurement system of the present invention.

FIG. 2 is a diagram of preferred Analog-to-Digital converters and theirinterface to the signal detector.

FIG. 3 is a functional block diagram of a preferred signal generator.

FIG. 4 depicts an implementation of a preferred Linear Feedback ShiftRegister.

FIG. 5 is a functional block diagram of a preferred signal detector.

FIG. 6 is a functional block diagram of a preferred frame accumulator.

FIG. 7 is a functional block diagram of a preferred frame correlator.

FIGS. 8A and 8B depict an embodiment of the present invention using a64-element photomultiplier array.

FIG. 9 is an embodiment of the present invention using an 11.times.11array of fibers to deliver light between the sources or detectors andthe patient.

FIG. 10 is a diagram illustrating the use of a 31-bit unipolar Galoiscode in one embodiment of the present invention.

FIG. 11 is a diagram illustrating the placement of source and detectorto measure total hemoglobin under one embodiment of the presentinvention.

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction withthese embodiments, it will be understood that they are not intended tolimit the invention to these embodiments. On the contrary, the inventionis intended to cover alternatives, modifications and equivalents, whichmay be included within the spirit and scope of the invention as definedby the appended claims. Furthermore, in the following detaileddescription of embodiments of the present invention, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. However, it will be recognized by one of ordinaryskill in the art that the present invention may be practiced withoutthese specific details. In other instances, well-known methods,procedures, components, and circuits have not been described in detailas not to unnecessarily obscure aspects of the embodiments of thepresent invention.

A functional block diagram of a preferred photon measurement system 100is depicted in FIG. 1. The photon measurement system can be used tomeasure the interaction of photons with a sample 5. In certainapplications, the sample 5 may be human breast tissue or fat tissue butit could just as well be any semitransparent material. The photonmeasurement system 100 preferably includes Temporal Response AnalysisEngine 11. The Temporal Response Analysis Engine 11 generates a digitalmodulation signal for driving an illumination light source that is usedto interrogate the sample. The Temporal Response Engine 11 also providesa means for processing a detected optical signal from the sample 5 toextract information about the sample 5. Preferably a digital modulationsignal 16 is generated in the signal generator 1 and transmitted to thetransmit signal conditioner 2. The digital modulation signal 16 is thedigital representation of a chosen code sequence. The code sequence ispreferably chosen from the known pseudorandom binary sequences (PRBS),Gold codes, Golay codes, Kasami codes, Walsh codes, or other codes thatpossess the preferred desirable property of large auto-correlationvalues and low cross-correlation values. The digital modulation signal16 may represent a single code pattern or multiple repeats of the samepattern. A single complete set of code patterns is designated amodulation frame or code pattern frame. The digital modulation signal 16is preferably transmitted to the signal detector 10 as an electronicreference signal 17. The transmit signal conditioner 2 formats thedigital modulation signal 16 as necessary to drive an opticalillumination source 3. In the preferred photon measurement system 100,the modulated optical source is a 785 nm diode laser made by HitachiCorp. Formatting of the digital modulation signal 16 in the preferredembodiment involves converting the digital modulation signal 16 to ananalog voltage waveform that is coupled through a 50-ohm bias-T to theDC drive current of the optical illumination source 3. In otherembodiments, the optical illumination source 3 may be a different laserdiode, a light-emitting diode, or a light source used together with anexternal optical modulator. The optical illumination source 3 generatesthe modulated optical wave 20 which is preferably transmitted to thesample 5 by light delivery optics 4. The preferred light delivery optics4 is a 3 mm diameter fiber bundle located between the opticalillumination source 3 and the sample 5 to deliver the modulated opticalwave 20 from the optical illumination source 3 to the sample 5. In otherembodiments the light delivery optics 4 comprises other arrangements ofoptical fibers, lenses, mirrors or other optical delivery components.When the modulated optical wave 20 illuminates the sample 5 scatteredoptical waves 21 are generated. In the preferred photon measurementsystem, the sample 5 is treated with a fluorescent material that willfluoresce when it is illuminated by the modulated optical waves 20. Inthe preferred system the scattered optical waves 21 are fluorescencegenerated from a fluorescent material within the sample 5. Thefluorescent material is preferably an exogenous contrast agent injectedinto the sample 5 or alternatively it is preferably some constituentcomponent of a material that exhibits endogenous fluorescence. Thedetection optics 6 are situated so that a portion of the modulatedoptical waves 21 are detected by the detection optics 6. In thepreferred photon measurement system 100, the detection optics 6 includean optical filter for separating the fluorescing scattered optical waves21 from the modulated optical waves 20. The optical filter preferablytransmits the higher wavelength fluorescence and blocks the lowerwavelength illumination light. In applications where the scatteredoptical waves 21 of interest are not fluorescing, an optical filter isnot required.

In the preferred photon measurement system 100, the detection optics 6preferably include a second 3 mm diameter fiber bundle located betweenthe optical filter and the optical detector 7. The optical detector 7converts the scattered optical waves 21 to an electronic signal. In thepreferred photon measurement system 100, the optical detector 7 ispreferably a photomultiplier tube, model R7400U-20 from Hamamatsu Corp.In other embodiments, the optical detector 7 may be a PIN photodiode, anavalanche photodiode, a charge-couple device, or other suitablephotosensitive element. As previously stated, the optical detector 7preferably converts detected scattered optical waves 21 into anelectronic signal which is communicated to the detected signalconditioner 8. The detected signal conditioner 8 preferably formats thesignal so it may be converted to discrete samples by an Analog toDigital (A/D) converter 9. The A/D converter 9 outputs a detectedresponse signal 19. The detected response signal 19 is communicated to asignal detector 10, where it is preferably correlated with theelectronic reference signal 17 to extract a sample transfercharacteristic.

Information about the temporal properties of the photons is preferablycalculated from the sample transfer characteristic. This informationpreferably includes such properties as direct measurements of photontime-of-flight and the fluorescence lifetime. The estimate of photontimes-of-flight is then preferably used to estimate characteristics ofthe tissue such as the absorption coefficient, scattering coefficient,or location of fluorescing material.

Another embodiment of the photon measurement system 100 includes anoptical reference generator 22. The optical reference generator 22preferably includes an optical splitter 12A or 12B that routes a portionof the modulated optical wave 20 to a secondary optical detector 13. Theposition of the optical splitter 12A or 12B can be either before orafter the light delivery optics. The output of the secondary opticaldetector 13 is preferably routed to a secondary signal conditioner 14whose output is communicated to a secondary A/D converter 15. Thesecondary A/D converter 15 preferably outputs a source reference signal18 which can be correlated with the detected response 19 to extract thesample transfer characteristic. Using the source reference signal 18 asopposed to the electronic reference signal 17 allows the filtering ofthe temporal properties of the signal conditioner 2 and the modulatedoptical source 3 from the measured transfer characteristic.

The preferred hardware implementation of the A/D converter module andits interfaces to the signal detector 10 are shown in FIG. 2. An arrayof N A/D converters 90 preferably receives the analog signal 95 inparallel from the signal conditioner 8 or 14. The output samples 18 or19 from the A/D converters 90 are preferably communicated to theFirst-In-First-Out buffers (FIFOs) 91 where they are buffered fordistribution to the internal components of the signal detector 10. Inthe preferred photon measurement system the A/D converters 90 are eightMAX 108 integrated circuits made by Maxim operating at 250 Msample/secand outputting two data samples at a time in parallel at 125 MHz. TheFIFOs 91 are preferably implemented within a Xilinx 4 FPGA. Theacquisition synchronizer 92 preferably controls signal acquisition anddigital data distribution through the conversion clock (CClk) signals96.

The acquisition synchronizer 92 is preferably synchronized with anexternally provided synchronization clock (SClk) 40 which is alsopreferably used to synchronize the signal generator 1. The signalsCClk[1 . . . N] are preferably generated within the acquisitionsynchronizer 92 and preferably have the same frequency as SClk 40 butare offset in phase from SClk 40 in N fixed increments of(360/N).degree., with the phase of CClk[1] set to the fixed offset ofZ.degree. In the preferred system the internal clock generationcapabilities of the Xilinx FPGA are used to implement the acquisitionsynchronizer 92 directly. The A/D converters 90 preferably perform theirconversions in sync with the conversion clocks 96 such that theygenerate samples at N discrete sample times spread evenly throughout thefundamental sample interval defined by the period of SClk 40. Theeffective sample rate for the array of converters is preferably N timesthe rate defined by SClk 40. This process of using multiple A/Dconverters sampling out of phase to increase the effective sample rateis what we call parallel over-sampling. In the preferred photonmeasuring system, parallel over-sampling results in an effective samplerate of 2 Gsamples/sec. The offset value Z allows the entire sample setto be offset by some phase from the synchronization clock 40. Theacquisition synchronizer 92 preferably is configured such that the valueof Z can be varied synchronously with the modulation frame, or with ablock of frames called a frame block. This allows Z to follow a sequenceof K values smaller than (360/N).degree. such that on successivemodulation frames/frame blocks the effective sampling phases (relativeto the synchronization clock) take on K values intermediate to thosecreated by the N conversion clocks in any given frame. In this casepreferably the input signal at any given A/D converter 90 will besampled at K discrete phases over K blocks. The detected response 19 ispreferably assumed to be stationary with respect to the start of thecode pattern block over that time interval. The preferred K discretesampling phases correspond to K discrete sample times and the effectivetemporal resolution of the sampling process is preferably increased by afactor of K. This process is referred to as temporal over-sampling.

In the preferred photon measuring system the value of Z is always zeroand temporal over-sampling is achieved by adjusting the phase of themodulation as described below rather than by adjusting the phase of theA/D converter sampling. Preferably the FIFOs latch input data to the A/Dconverters 90 synchronously with the corresponding conversion clock 96.The FIFO 91 output data is preferably provided to the internalcomponents of the signal detector 10 synchronously with thesynchronization clock 40 such that all further processing issynchronized with the synchronization clock 40.

The preferred implementation of the Temporal Response Analysis Engine 11are shown in FIGS. 3 through 7; the preferred signal generator 1 isshown in FIGS. 3 and 4, while the preferred signal detector 10 is shownin FIGS. 5, 6, and 7. In the preferred system the Temporal ResponseAnalysis Engine 11 is implemented as logic blocks within a Xilinx 4FPGA.

The functional blocks of the preferred signal generator 1 are shown inFIG. 3. The top 41 and bottom 42 signal paths are two preferred variantsfor generating different code patterns for the modulation signal 16. Inthe top path 41 a Linear Feedback Shift Register (LFSR) 30 is preferablyused to create a PRBS code. The specific code pattern is preferablydetermined by the number of state bits within the LFSR 30 and the gaincode 36 input to the LFSR 30. In one preferred implementation the gaincode 36 is stored in a gain memory 31, which is preferably configured toallow the code pattern 16 to be changed during operation either byselecting one of several gain codes from a read-only memory or bysetting a new gain code into a writable memory. In other embodiments thegain code 36 may be hard-wired into the LFSR 30, or a code-specificstate-machine designed to generate a desired code through a series ofstate transformations may be used in place of the LFSR 30. In the bottompath 42 the entire code pattern is preferably stored as a bit sequencein a pattern memory 32. The sequence in which pattern bits are presentedis preferably determined by an address sequencer 33 which preferablyprovides the cell addresses 37 for the memory. The address sequencer 33is preferably configured to allow changing the code pattern 16 duringoperation either by selecting one of several patterns stored in aread-only memory or by inputting a new pattern into a writable memory.

The modulation signal 16 for both the LFSR 30 or pattern memoryimplementation is preferably buffered by an output buffer 35 to make thesignals 16 more robust when driving external components. Timing forpresentation of the code pattern bits is preferably controlled by ageneration synchronizer 34 which preferably generates the master clock(MClk) 38 for the LFSR 30 and the address sequencer 33. The master clock38 is preferably synchronized to a system synchronization clock (SClk)40 which preferably controls both code pattern generation and responsesignal acquisition. MClk 38 preferably operates at the same frequency asSClk 40 but is preferably offset in phase by an amount specified by thephase input 39, which is preferably an externally programmableparameter. This phase offset allows the relative phase between themodulation signal 16 and the detected response 19 to be adjusted. If thephase is adjusted by some increment, (360/K).degree., at the end of eachcode pattern block or set of blocks the detected response resulting fromthe modulation signal will preferably be sampled at K discrete phasesover K blocks. In this embodiment of the photon measuring system as withthe preferred embodiment, the detected response 19 is assumed to bestationary with respect to the start of the code pattern block over thattime interval so that the K discrete sampling phases correspond to Kdiscrete sample times and the effective temporal resolution of thesampling process is increased by a factor of K.

This temporal over-sampling is functionally equivalent to the techniquedescribed for temporal over-sampling in the A/D converter embodiment. Inother embodiments the external phase specification may represent thephase increment rather than the absolute phase, and the generationsynchronizer 34 may increment the phase internally.

The preferred implementation of the LFSR 30 is shown in FIG. 4. The LFSR30 is preferably a state-machine comprising M standard LFSR cells 48which hold and transform the state. The LFSR cells 48 are preferablylinked in a numbered sequence, and the output from the LFSR 30 is thecurrent state of cell number zero. Each cell preferably comprises astate latch 45 which holds a single bit of state information, a gainelement 46 to control the feedback gain for the cell based on theexternally provided gain code 36, and an accumulator 47. The accumulator47 preferably adds the feedback from the cell to the cumulative feedbackfrom all previous cells. At each clock increment the state for a cell isupdated to match the previous state from the next higher cell in thechain; the state of the last cell in the chain is updated with theaccumulated feedback from all the previous cells. The accumulator 47 forthe last cell in the chain may be omitted if desired. The patterngenerated by the LFSR 30 is preferably determined by the number of cellsin the chain and by the gain code. In a preferred embodiment the gaincode is provided from an external source to allow the code pattern to bemodified. In other embodiments the gain code may be a fixed value. Inembodiments in which the gain code is fixed, the implementation of thegain elements and accumulators for each cell may be optimized for thespecific gain code for that cell rather than implemented in thegeneralized fashion shown. The clock for the LFSR 30 and for all itsinternal latches is preferably the signal generator master clock 38.

The preferred functional blocks for the signal detector 10 are shown inFIG. 5. The detected response 19 and either the electronic referencesignal 17 or the source reference signal 18 are received at two frameaccumulators 50 and 51, where the samples for each discrete sample timeare accumulated with samples from identical sample times from differentmodulation frames to form the aggregated detected response 58 and theaggregated reference signal 59. As a result of this aggregation, theeffective data rate at which samples are preferably processed infollowing blocks is reduced by a factor equal to the number of framesaggregated into each sample point. The frame accumulators 50 and 51 arepreferably replicated N times to handle the N channels of the A/Dconverter independently. The internal details of the frame accumulators50 and 51 for the detected response and the reference signal may differ,depending on the digital format of the two signals. For example, if thereference signal used for analysis is the electronic reference signal 17rather than the source reference signal 18 its value for each sampletime is known a priori to be identical for every frame and to take ononly two possible binary values, 0 or 1. In that case preferably theframe accumulator 51 for the reference signal 17 need only store one bitper sample time, equal to the value of the modulation signal for thatsample time. At some point between the output of the frame accumulatorsand final output of the sample transfer characteristic 57 the Nacquisition/accumulation channels are preferably re-interleaved into asingle data stream. In one preferred embodiment two multiplexers 52 and53 perform this reintegration at the output of the frame accumulators 50and 51. In other embodiments this re-integration may take place at anyother desired point in the signal processing chain. With or withoutre-integration the aggregated detected response 58 and the aggregatedreference signal 59 are routed to the frame correlator 55 where the twosignals 58 and 59 are preferably combined by a cross-correlationalgorithm to produce the correlated signal 61 which preferably comprisesa single value for each complete aggregated frame of samples. Thecorrelated signal 61 represents the degree to which the aggregatedresponse signal 58 contains components matching the aggregated referencesignal 59. If the aggregated reference signal 58 is delayed by a time.tau. before presentation to the correlator 55 then the correlatedsignal 61 represents the degree to which the aggregated response signal58 contains components of the delayed version of the reference signal60. The sample transfer characteristic 57 comprises a set of correlatedsignals calculated for a range of J such delay times. Phase delay blocks54 generate the delayed versions of the aggregated reference signal 60.For simplicity the J phase delay blocks 54 are illustrated as discretecomponents operating in parallel and each providing the complete delayrequired for one correlated signal. In one preferred embodiment theycomprise a cascade of J phase delay blocks each providing the timeincrement between one correlated signal and the next. The phase delaysfor the correlated signals are preferably discrete and correspond tointegral multiples of the synchronization clock 40 period. The phasedelay blocks 54 are preferably implemented as shift registers or FIFOsof the appropriate depth and clocked by the synchronization clock 40. Inother embodiments the time delay may be implemented using other methods.In one preferred embodiment each phase delay is processed by acorresponding frame correlator 55. In other embodiments a single framecorrelator 55 may be used to calculate the correlated signal 61 formultiple phase delays by presenting the detected response data to itsinput multiple times, using a different phase delayed version of thereference signal 60 for each iteration. In this case fewer framecorrelators 55 are required.

The details of the preferred frame accumulator 50 or 51 are shown inFIG. 6. Samples from the signal 17, 18, or 19 are preferably accumulatedin the adder 70 by summing them with values taken from the memory 71;the resulting aggregated signal 58 or 59 is routed to the output of theaccumulator and stored back into the memory at the same location fromwhich the original data was taken. Each discrete sample time for thechannel is represented by a single addressed cell within the memory. Thesize of the memory is preferably determined by two parameters, K and R,which preferably encode the sampling scheme. K represents the number ofdiscrete phases at which samples are preferably taken in various framesduring temporal over-sampling. R is the ratio of the number of samplesin a modulation frame to the number of sampling channels provided in theA/D converter 90 for parallel over-sampling and signifies the number ofsamples that must be accommodated by each channel within a single frame.A preferred sample enable gate 72 is provided to restart theaccumulation process at the beginning of each set of frames by clearingthe cells in the memory. The address sequencer 73 selects the cell ofthe memory to be addressed for each sample point. The frame accumulators50 or 51 preferably run synchronously with the synchronization clock 40(although out of phase), so only a single address sequencer is requiredto address all the frame accumulators.

The details of the preferred frame correlator 55 are shown in FIG. 7.The ideal method for correlating the signals is to take the integral ofthe detected response 19 weighted by the reference signal 17 or 18.Because the preferred embodiment is a sampled system the integration isapproximated by summation over all the samples within a frame set usingthe adder 81 to generate the correlation signal 61. The weighting of theaggregated detected response 58 by the aggregated reference signal 59 ispreferably performed by a multiplier 80. Other embodiments may employother weighting and integration schemes, including scaling andintegration in the analog domain directly on the detected signals. Asample enable gate 82 is preferably provided to restart the accumulationprocess at the beginning of each set of frames by clearing thecorrelator.

The photon measurement system 100 is useful for interrogating a sectionof tissue located generally between the light delivery optics and thedetection optics. In order to interrogate a larger tissue volume, it isuseful to have a system where the photon measurement system isreplicated so that separate tissue sections can be interrogated withseparate source-detector pairs. One embodiment of such a system is shownin FIGS. 8A and 8B. Eight fiber bundles 85 are used to deliver lightfrom eight different sources to the tissue. The fiber bundles are shownencircled by the dotted line in FIG. 8A. The detectors are a 64-elementphotomultiplier array 86 manufactured by Hamamatsu with the individualelements in an 8.times.8 arrangement. Fluorescent light from the tissuepasses through an optical filter 88 that blocks light at the excitationwavelength. The fluorescent light is coupled to the detector array by a2.5:1 tapered imaging fiber bundle 87 made by Schott Corp. An explodedview of the detector array 86, filter 88, and imaging fiber bundle 87 isgiven in FIG. 8B. Each source-detector pair can be coupled toelectronics as shown functionally in FIG. 1 to form an individual photonmeasurement system. Each source-detector pair yields information aboutphoton time of flight through a somewhat different section of tissuethan any other pair. Each source can be turned on sequentially, whileall the detectors can be sampled simultaneously while a given source ison. Alternatively, each source can be driven with a different code suchthat any code is orthogonal to the others. In this case, the sources canbe driven simultaneously and the low cross-correlation of the respectivereference signals allows separation of the signals. The sequential casewill exhibit improved signal-to-noise ratio compared to thesimultaneously on case due to the non-ideal cross-correlations obtainedin practice. Another embodiment of the present invention is shown inFIG. 9. In this case, the imaging instrument 110 includes an 11.times.11array 91 of multimode fibers for coupling light from the sources anddetectors in an electronics module 92 to the tissue. Each fiber can becoupled to either a source or a detector. The fibers are spaced at 1 cmintervals on the imaging head 90. The image reconstructed from themeasured data is displayed on the monitor 93. The imaging head 90 caneasily be manipulated to image various parts of the patient 94. Thepresent invention is not limited to the particular geometries describedhere. The use of the photon measurement system 100 is possible withvarious combinations of sources and detectors and various positions ofthe sources and detectors. In the examples described, the geometry is areflection geometry with the sources and detectors effectively on thesame side of the tissue. In other embodiments, the detection optics canbe placed on the opposite side of the tissue from the light deliveryoptics. The particular number of sources and detectors can also bevaried depending on the resolution and field-of-view required for aparticular application. In the present embodiments, the instrument isintended to cover an area of approximately 10 cm.times.10 cm area.Imaging a larger area can be accomplished by moving the instrument headacross the area. The embodiments described utilize a photomultiplierarray as the optical detectors. In other embodiments, it is possible touse PIN photodiodes, avalanche photodiodes, individual photomultipliertubes, detector arrays, charge-coupled device arrays, or otherphotosensitive elements.

The present invention is utilized for sentinel lymph node mapping aspresently described. A patient is injected near the site of a malignancywith a dye that fluoresces when exposed to near-infrared light. Inparticular, indocyanine green (ICG) can be excited at wavelengths around785 nm and fluoresces at wavelengths around 830 nm. The dye serves bothas a visual guide for the surgeon and as a contrast agent for theoptical imaging system. ICG has the advantage that it is alreadyapproved for use in medical procedures such as angiography; however,several alternative contrast agents are also available. Imaging proceedsas follows. Assuming the imaging is performed reasonably soon afterinjection of the dye, the dye will be relatively well-localized in thesentinel node or nodes. If the dye is ICG, this amount of time is onethe order of minutes. The imaging head is placed in contact or in closeproximity to the tissue suspected of containing the sentinel node. Thecorrelator output, or characteristic transfer function, is measured foreach source-detector pair. For any given source and detector position,it is possible to calculate a priori the expected characteristictransfer function for a given location of fluorescence dye. In practice,because the tissue is so highly scattering, neighboring source-detectorpairs can have somewhat overlapping interrogation regions. The imagereconstruction problem consists of estimating the most likelydistribution of dye given all the measurements of characteristictransfer functions from all the source-detector combinations. Varioustechniques are known for performing such an inversion problem, includingsuch methods as singular-value decomposition and the AlgebraicReconstruction Technique, also known as the Gauss-Seidel method. Theresult of the inversion is a volumetric map of the location of dyewithin the tissue. Because the dye collects predominantly in thesentinel node(s), this map is effectively a map of the sentinel nodelocation. This map is displayed in the form of an image or images on amonitor attached to the instrument. The surgeon uses this image to planhis surgical incisions. The estimated positions of the sentinel nodewith respect to the instrument are also displayed on the monitor,allowing the surgeon or other operator to mark the body before thesurgery begins.

A preferred imaging method for locating the sentinel lymph node or nodesis as follows. The patient is injected with fluorescent material nearthe site of a malignancy. Imaging begins after an amount of timesufficient for the fluorescent material to reach the sentinel lymph nodeor nodes. The instrument head is placed over the patient at a positionthat represents an initial estimate for the location of the sentinelnode. With the instrument head in position, the first optical source isturned on for an amount of time corresponding to the desired number ofrepeats of the code sequence. Scattered optical waves are measured ateach corresponding detector. The output of each detector is correlatedwith the reference signal as described above to produce a temporaltransfer characteristic corresponding to the source-detectorcombination. The temporal transfer characteristics for eachsource-detector combination are stored in memory. The process isrepeated for each subsequent optical source until temporal transfercharacteristics are collected for all desired source-detector pairings.The acquired temporal transfer characteristics are then used toreconstruct an image of the underlying tissue volume using an algorithmimplemented in software. The algorithm is based on the ability toestimate a priori the temporal transfer characteristic that will beobtained for any source-detector pairing for any particular location offluorescent dye. The algorithm generates a most likely estimate of thefluorescent material locations based on the a priori models given themeasured temporal transfer characteristics. This estimate of fluorescentmaterial locations is displayed in the form of a volumetric image on amonitor connected to the instrument. The user of the instrument canconclude based on the image whether or not the underlying tissuecontains a sentinel node. Generally, the node will be imaged as a subsetof the volume with a high estimated concentration of fluorescentmaterial. If the user judges that the sentinel node has been located, hemay physically mark the body where the instrument head had been placedwith a pen to indicate the area in which to cut. Alternatively, he maysave the image on the screen or on a printout so that it may be referredto during surgery. If the user concludes that the sentinel lymph nodehas not been located, he moves the instrument to a different locationand the process is repeated.

Under an embodiment of the present invention, Temporal Response AnalysisEngine 11 comprises a general purpose microprocessor. Temporal ResponseAnalysis Engine 11 can also comprise software which providesinstructions to the microprocessor. Alternatively, Temporal ResponseAnalysis Engine 11 can comprise an embedded processor or otherprocessor, application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA) or other integrated circuits. TemporalResponse Analysis Engine 11 can also comprise firmware.

Under another embodiment of the present invention, the chosen codesequence of digital modulation signal 16 is a unipolar code sequence.Digital modulation signal 16 can be transmitted to optical illuminationsource 3 or can be used with an external modulator or electro-opticmodulator with optical illumination source 3. A unipolar code sequenceallows for the use of commercially available continuous wave lasers orlight emitting diodes (LEDs) as a light source, for example as opticalillumination source 3. A bipolar code sequence does not allow for theuse of commercially available continuous wave lasers or LEDs as a lightsource because a bipolar code sequence requires the transmission of −1'sor negative states. With commercially available continuous wave lasersor LEDs, −1's or negative states are difficult to achieve. In addition,the chosen code sequence of digital modulation signal 16 can be a codesequence where the autocorrelation is orthogonal. An orthogonal codesequence can result in a correlation which is flat or relatively flataway from both sides of the peak and can make the processing andanalysis for the temporal transfer characteristic or the temporal pointspread function easier as well as reducing errors. This characteristicalso allows for simultaneous transmission of multiple code sequences andanalysis of the multiple code sequences without interference from eachcode sequence.

Under another embodiment, the chosen code sequence of digital modulationsignal 16 is a code sequence with high autocorrelation approaching thedelta function and low cross-correlation values. The chosen codesequence can be an Optical Orthogonal Code. Two codes of length N=36 or36 elements can be used, 11010001000000000000000000000000 and10000100000001000000000010000000. The maximum autocorrelation value is 4and the maximum cross-correlation value is 1. The ratio of the maximumautocorrelation value to maximum cross-correlation value is 4. However,Optical Orthogonal Codes generally have many more 0s (or low states)than 1s (or high states) making them difficult to implement withcommercially available continuous wave lasers or LEDs. In addition, therelatively high cross-correlation values hinder the processing andanalysis for the temporal transfer characteristic or the temporal pointspread function and can introduce errors.

Under another embodiment, the chosen code sequence of digital modulationsignal 16 comprises individual code elements where the individual codeelements have a length of one nanosecond. Alternatively, individual codeelement lengths of 25 ps, 50 ps, 75 ps, 100 ps, 125 ps, 150 ps, 175 ps,200 ps, 250 ps, 500 ps, 750 ps, 1 ns, 1.5 ns, 2 ns, 2.5 ns, 3 ns, 4 ns,5 ns, 6 ns, 7 ns, 8 ns, 9 ns, 10 ns, 11 ns, 12 ns, 13 ns, 14 ns, 15 ns,16 ns, 17 ns, 18 ns, 19 ns, 20 ns or any length in between such lengthsor any range of lengths in between 25 ps and 20 ns. could be used.Individual code element lengths that are longer allow the use of slowerand less expensive lasers or LEDs for optical illumination source 3.However, the amount of time to transmit and process the chosen codesequence of digital modulation signal 16 is dependent on the individualcode element lengths multiplied by the number code elements in eachsequence. In addition, the width of the temporal transfer characteristicor the temporal point spread function can be as narrow as one nanosecondor less. For narrow temporal transfer characteristics or the temporalpoint spread functions, a long code element length would lack adequateresolution to properly derive the temporal transfer characteristic orthe temporal point spread function.

Under another embodiment, multiple code sequences of 2, 3, 4, 5, 6, 7,8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 1,000, 10,000, 100,000,1,000,000 code sequences or any code sequence in between such codesequences or any range of code sequences in between 2 and 1,000,000 codesequences can be used and correlation performed on averaged data oraverage performed on correlations of data. The multiple code sequencescan be multiple identical code sequences. Use of multiple code sequencesallows photon measurement system 100 to average out noise effects,improve signal-to-noise ratio, temporary deviations in the system or thesample or average out data prior to stabilization of opticalillumination source 3. However, a long individual code element lengthcan result in long processing time particularly for high number of codeelements in each sequence and particularly if a large number of multiplecode sequences is utilized.

Under another embodiment, the chosen code sequence of digital modulationsignal 16 is a code sequence from the Golay class of codes. Golay codesare bipolar making them difficult to use with commercially availablecontinuous wave lasers or LEDs as a light source. However, in thisembodiment, the bipolar Golay code sequence is converted into twounipolar code sequences. For example, a bipolar code sequencerepresented by A(t) can take on values 1 and −1. Two unipolar codesequences UA1(t) and UA2(t) can be constructed where UA1(t)=1/2[1+A(t)]and UA2(t)=1/2[1−A(t)].

In addition, complementary Golay codes can be used where the sum of theautocorrelations is a delta function with the maximum autocorrelationvalue equal to N where N is the length of the code sequence or thenumber of individual code elements in the code sequence. In thisexample, the bipolar code sequence represented by A(t) can be convertedto two unipolar code sequences UA1(t) and UA2(t) whereUA1(t)=1/2[1+A(t)] and UA2(t)=1/2[1−A(t)]. The complementary bipolarcode sequence represented by B(t) can be converted to two unipolar codesequences UB1(t) and UB2(t) where UB1(t)=1/2[1+B(t)] andUB2(t)=1/2[1−B(t)]. Four code sequences UA1(t), UA2(t), UB1(t) andUB2(t) would be used to drive optical illumination source 3. Fourreadout traces could be obtained RA1(t)=UA1(t)*f(t), RA2(t)=UA2(t)*f(t),RB1(t)=UB1(t)*f(t), and RB2(t)=UB2(t)*f(t). The temporal transfercharacteristic or the temporal point spread function can be obtained byperforming the following calculation:fest=A(t)·[RA1(t)−RA2(t)]+B(t)·[RA1(t)−RA2(t)]. Using the four unipolarcode sequences has the advantage that commercially available continuouswave lasers or LEDs can be utilized as a light source, for example asoptical illumination source 3. In addition, the sum of theautocorrelations approaches a delta function where width is related tocode element length, making it easier to derive the temporal transfercharacteristic or the temporal point spread function. However, usingfour code sequences has the disadvantage that longer transmission timeand longer processing time is required. If optical illumination source 3is unstable or exhibits amplitude variations or different DC biases,errors can be introduced in processing and processing can be moredifficult. In addition, because each code sequence can result in adifferent DC bias and optical illumination source 3 may require a periodof stabilization during each code sequence, the stabilization wouldintroduce additional transmission time and processing time for each codesequence.

Under another embodiment, the chosen code sequence of digital modulationsignal 16 is a code sequence from the Galois class of codes. Galoiscodes do not have ideal autocorrelation but the autocorrelation isuniform on both sides of the peak. The uniformity allows for better orenhanced noise processing and enhanced ability to derive the temporaltransfer characteristic and the temporal point spread function. Galoiscodes have the advantage that it can be implemented with a singleunipolar code sequence. The single unipolar code sequence makes photonmeasurement system 100 less susceptible to instability, amplitudevariations or differing DC biases in optical illumination source 3. Inaddition, to the extent optical illumination source 3 may require aperiod of stabilization during each code sequence, the stabilizationtime would have less of an impact on transmission time and processingtime. The chosen code sequence of digital modulation signal 16 using acode sequence from the Galois class of codes has a circularautocorrelation of N or approaching N near the peak and −1 orapproaching −1 away from the peak, where N is the length of the codesequence or the number of individual code elements in the code sequence.The ratio of the maximum circular autocorrelation value to maximumcross-correlation value is N. A circular code sequence has the importantfeature that the circular autocorrelation can begin at any point or anycode element. The phase of the code sequence does not need to betracked. The chosen code sequence of digital modulation signal 16 usinga code sequence from the Galois class of codes can have 31, 63, 127,255, 511, 1023, 2047, 4095 and 8191 individual code elements in the codesequence.

Under another embodiment, the chosen code sequence of digital modulationsignal 16 is a linear-feedback shift-register sequence, in particular amaximal-length sequence or m-sequence. An n-bit shift register canencode 2^(n) states, so an m-sequence or maximal-length sequence canhave 2^(n−1) elements before repeating. All zeros in the shift registeris a fixed-point unto itself so it cannot be part of any sequence longerthan 2^(n−1). Maximal-length sequences or m-sequences have one more 1'sthan 0's. The circular autocorrelation of a maximal-length sequence orm-sequence with itself has one value of 2⁻¹ at zero lag and the rest ofthe values equal to 2^(n−2). Although the non-zero value at the otherlag is undesirable, it results in a finite transmission of a DCcomponent through the system which can be removed through filtering. Abipolar sequence comprising 1's and −1's can have betterautocorrelation. However, −1's require phase sensitive detection.

Under another embodiment, multiple identical code sequences of 2, 3, 4,5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 1,000, 10,000,100,000, 1,000,000 code sequences or any code sequence in between suchcode sequences or any range of code sequences in between 2 and 1,000,000code sequences can be used. The detected response signal 19 resultingfrom the entire set of multiple identical code sequences is correlatedwith the electronic reference signal 17 or source reference signal 18 ofthe entire set of multiple identical code sequences. The multipleidentical code sequences can be periodic or circular. Use of periodic orcircular multiple identical code sequences, particularly for Galoisclass of codes, results in high autocorrelations approaching the deltafunction and low cross-correlation. This characteristic allows forsimultaneous transmission of multiple code sequences and analysis of themultiple code sequences without interference from each code sequence.Each code sequence can be a separate channel and can start at differenttimes. In addition, the autocorrelation of a single code sequence or thecorrelation of detected response signal 19 resulting from a single codesequence with the electronic reference signal 17 or source referencesignal 18 of a single code sequence can result in significantside-lobes. The side-lobes hinder the processing and analysis for thetemporal transfer characteristic or the temporal point spread functionand can introduce errors. Use of periodic or circular multiple identicalcode sequences can significantly reduce or eliminate the side-lobes inthe autocorrelation or correlation. However, a long individual codeelement length can result in long processing time particularly for highnumber of code elements in each sequence and particularly if a largenumber of multiple code sequences is utilized.

Under another embodiment, radio frequency (RF) shielding is applied tothe components of photon measurement system 100. Certain components inphoton measurement system 100, for example, signal generator 1, signalconditioner 2, or optical illumination source 3, can generate noisewhich can appear at optical detector 7, A/D converter 9 or signaldetector 10. This noise can then appear in the temporal transfercharacteristic or the temporal point spread function making it difficultto analyze or introducing errors for photon time-of-flight, fluorescencelifetime, tissue absorption coefficient, tissue scattering coefficient,location of fluorescing material or other tissue properties orcharacteristics. Signal generator 1, signal conditioner 2, or opticalillumination source 3 can be RF shielded to reduce or avoid noiseappearing at optical detector 7, A/D converter 9 or signal detector 10.

Alternatively or concurrently, a delay component or element can beplaced between optical illumination source 3 and optical splitter 12A,between optical splitter 12A and optical detector 13, between opticalsplitter 12B and optical detector 13, between sample 5 and detectionoptics 6 or between detection optics 6 and optical detector 7. The delaycomponent or element can be a length of free space or a length ofoptical fiber, optical waveguide or optical bundle. Optical fiber,optical waveguide or optical bundle can be dispersive, both spectral andtemporal, can propagate multimodes in the cladding which can distort theoptical signal. Free space has the advantage of causing less distortionto the optical signal. A single mirror or 2, 3, 4 or 5 mirrors can beused. Alternatively, retroreflectors, prisms, reflectors or otherreflective surface can be used. Use of a reflective surface or aplurality of reflective surfaces allows a given length of free space tooccupy significantly less physical dimension and be more compact. Asingle reflective surface can allow the light to travel back along itsoriginal path. In this manner, the physical length can be reduced up tofifty percent. The physical length can be further reduced by usingmultiple reflective surfaces. With two reflective surfaces, the lightcan travel along the same path three times, reducing the physical lengthby up to 66⅔ percent. With three reflective surfaces, the light cantravel along the same path four times, reducing the physical length byup to 75 percent. With four reflective surfaces, the light can travelalong the same path five times, reducing the physical length by up to 80percent. With five reflective surfaces, the light can travel along thesame path six times, reducing the physical length by up to 83⅓ percent.Alternatively, instead of using three reflective surfaces, tworeflective surfaces can be used with the light reflecting off of onereflective surface twice and travelling along the same path four times.Instead of four reflective surfaces, two reflective surfaces can be usedwith the light reflecting off of each reflective surface twice andtravelling along the same path five times. Instead of five reflectivesurfaces, two reflective surfaces can be used with the light reflectingoff of one reflective surface twice and one reflective surface threetimes, travelling along the same path six times.

The amount of delay resulting from the delay component or element can beadjusted by altering the length or by material selection of materialswith differing index of refraction. The delay causes the noise toseparate from the temporal transfer characteristic or the temporal pointspread function after correlation of detected response signal 19 withthe electronic reference signal 17 or source reference signal 18. Theseparation of noise from the temporal transfer characteristic or thetemporal point spread function aides analysis and reduces errors forphoton time-of-flight, fluorescence lifetime, tissue absorptioncoefficient, tissue scattering coefficient, location of fluorescingmaterial or other tissue properties or characteristics. The amount ofdelay can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15nanoseconds or greater or any delay amount in between such delay amountsor any range of delay amounts in between 1 nanosecond and 15nanoseconds. A greater amount of delay would require a delay componentor element of greater length.

Under another embodiment, photon measurement system 100 is used tomeasure oximetry in human tissue or more specifically, noninvasive humancerebral oximetry. Photon measurement system 100 can comprise a secondoptical detector associated with a second optical transmission path fromoptical illumination source 3 through sample 5. The first opticaldetector, i.e. optical detector 7, can be associated with a firstoptical transmission path through sample 5, e.g. through the human scalpand skull only. The second optical detector can be associated with asecond optical transmission path e.g. through the human scalp, skull andbrain. Tissue characteristics can be derived for the brain alone bycomparing the optical signal from the first optical transmission pathwith the second optical transmission path.

Light attenuation through tissue can be described by the Beer-Lambertlaw:

I=I _(o) exp[−(μ_(a)+μ_(s)′)l]  (Eqn 1)

where I is the measured light intensity after passing through themedium, I₀ is the initial light intensity, μ_(a) is the absorptioncoefficient and μ_(s)′ is the reduced scattering coefficient, and l isthe optical path length through the medium. This equation can berewritten as:

$\begin{matrix}\begin{matrix}{{- {\log \left( \frac{I}{I_{o}} \right)}} = \frac{\left( {\mu_{a} + \mu_{s}^{\prime}} \right)l}{2.303}} \\{= {{ɛ_{1}C_{1}l} + {ɛ_{2}C_{2}l} + {\ldots \mspace{14mu} ɛ_{n}C_{n}l} + \frac{\mu_{s}^{\prime}l}{2.303}}}\end{matrix} & \left( {{Eqn}\mspace{14mu} 2} \right)\end{matrix}$

where n is the number of absorbing species, ε is the molar absorptivity(also known as the molar extinction coefficient), and C is theconcentration of the absorbing species. The absorption coefficient andmolar absorptivity are wavelength-dependent and characteristic of aparticular molecule. The molar absorptivities for many compounds can bereadily determined.

From Eqn 2, the following can be determined:

μ_(a)=2.303(ε₁ C ₁+ε₂ C ₂+ . . . ε_(n) C _(n))   (Eqn 3)

In other words, from the absorption coefficient, concentration can bedetermined without need of optical pathlength. At least n wavelengths oflight are required to identify any one absorber of light out of a systemof n absorbers.

The primary absorbers in human tissue and blood are oxyhemoglobin andreduced hemoglobin (also known as deoxyhemoglobin). Water is the nextstrongest absorber. Therefore, to determine functional oxygensaturation, which is defined as

$\begin{matrix}{{O_{2}\mspace{14mu} {sat}\mspace{14mu} \%} = {\frac{{{Hb}O}_{2}}{{Hb} + {{Hb}O}_{2}} \times 100\%}} & \left( {{Eqn}\mspace{14mu} 4} \right)\end{matrix}$

a minimum of 2 wavelengths, preferably 3 to account for the combinedeffect of water and other absorbers should be used. These 3 wavelengthsshould fall in the range of 650 nm to 1000 nm, preferably (1) 740 nm to770 nm, preferably 760 nm; (2) 770 nm to 820 nm, preferably 805 nm(isosbestic point); (3) 820 nm to 1000 nm, preferably 850 nm.

The temporal transfer characteristic or temporal point spread functionof the tissue can be extracted from the temporal response profile byusing of the Temporal Response Analysis Engine 11. The tissue temporaltransfer characteristic or temporal point spread function can be fitwith diffusion theory or similar to extract the absorption coefficient,μ_(a), independently from the scattering coefficient, optical pathlength, or other parameters. Alternatively, μ_(a) can be found bycorrelation with other statistical measures of the temporal transfercharacteristic or temporal point spread function such as moments of thedistribution, peak width at various fractional peak heights, peak area,or by fitting a linear slope to the tail of the profile. Once μ_(a) hasbeen determined at each selected wavelength, Eqn 3 can be used to findthe concentrations of oxyhemoglobin, deoxyhemoglobin and, if desired,water and other absorbers. The resulting concentrations can then be usedin Eqn 4 to calculate oxygen saturation.

Using this technique, the measured concentrations of hemoglobin (andderivatives) can be absolute and accurate, without influence from tissuescattering or variations in optical path length. The oxygen saturationvalue calculated using these absolute concentrations can also beabsolute and accurate.

Photon measurement system 100 can further comprise a second opticalillumination source operating at a second wavelength. The firstwavelength and second wavelength can be used to determine the amount ofoxygenated hemoglobin and deoxygenated hemoglobin. Alternatively, photonmeasurement system 100 can further comprise a third optical illuminationsource operating at a third wavelength. The third wavelength can be usedto determine the contribution of water or other absorbers to obtain moreaccurate measurement of oxygenated hemoglobin and deoxygenatedhemoglobin. Alternatively, photon measurement system 100 can furthercomprise a fourth optical illumination source operating at a fourthwavelength. The fourth wavelength can be used to determine the amount ofcarboxyhemoglobin. Alternatively, photon measurement system 100 canfurther comprise a fifth optical illumination source operating at afifth wavelength. The fifth wavelength can be used to determine theamount of methemoglobin. A single optical detector can be used for eachwavelength. However, many optical detectors would be requiredparticularly if multiple optical transmission paths are involved. In theexample of two wavelengths and two optical transmission paths for eachwavelength, four optical detectors would be required. In the example ofthree wavelength and two optical transmission paths for each wavelength,six optical detectors would be required. However, the difficulty existsof separating and deriving the temporal transfer characteristic or thetemporal point spread function for each wavelength since the outputsignal from the optical detector will represent the combination ofmultiple wavelengths. In addition, use of multiple detectors can requirethe use of an optical filter to separate wavelengths, adding loss.

Alternatively, a single optical detector could be used for multiplewavelengths. In the example of two wavelengths and two opticaltransmission paths, two optical detectors would be required instead offour. In the example of three wavelengths and two optical transmissionpaths, two optical detectors would be required instead of six. Inaddition, it can still present difficulties especially if wavelengthsare close in spectrum to each other resulting in incomplete separation.Photon measurement system 100 or Temporal Response Analysis Engine 11can further comprise a separate signal generator for a wavelength or anoptical illumination source. The timing of the initiation of the chosencode sequence of the digital modulation signal for multiple signalgenerators can be delayed. The initiation delay can cause the temporaltransfer characteristic or the temporal point spread function for theassociated wavelength or associated optical illumination source to bedelayed with respect to another wavelength or optical illuminationsource. This delay can result in separation of the temporal transfercharacteristic or the temporal point spread function for individualwavelengths making it easier to distinguish the temporal transfercharacteristic or the temporal point spread function for individualwavelengths. Alternatively, the same result can be achieved by usingdifferent code sequence for separate signal generators. Code sequencecould be chosen that result in separation or delay of the temporaltransfer characteristic or the temporal point spread function fordifferent wavelengths. The separation or delay can also be implementedby placing a delay component or element between optical illuminationsource 3 and optical splitter 12A, between optical splitter 12A andoptical detector 13, between optical splitter 12B and optical detector13, between sample 5 and detection optics 6 or between detection optics6 and optical detector 7. The delay component or element can be a lengthof free space or a length of optical fiber, optical waveguide or opticalbundle.

The separation or delay can be characterized in terms of time or numberof code elements. The amount of time for the separation or delay can becalculated as the number of code elements for the separation or delaymultiplied by the length of the individual code element. The separationor delay can also be characterized in terms of fractions or percentageof the number of individual code elements in each code sequence. Theamount of separation or delay can be set by starting the code sequenceat a different point for each wavelength. As an example, if twowavelengths are used with a code sequence of 31 individual elements andindividual code element length of 1 nanosecond, the first wavelengthcould be transmitted starting with the first code element and the secondwavelength could be transmitted starting with the 15^(th) or 16^(th)code element. The amount of separation or delay between the firstwavelength and second wavelength in this example would be 14 nanosecondsand 15 nanoseconds for transmission starting with 15^(th) and 16^(th)code element, respectively. Starting the second wavelength at the15^(th) or 16^(th) code element provides maximum amount of separation ordelay between first wavelength for code sequence of 31 individualelements. Increased amount of separation or delay allows the temporaltransfer characteristic or the temporal point spread function fordifferent wavelengths to be more easily distinguished from one another.For code sequence of 63 individual elements, starting the secondwavelength at the 31^(st) or 32^(nd) code element provides maximumamount of separation or delay between the first wavelength. For codesequence of 127 individual elements, starting the second wavelength atthe 63^(rd) or 64^(th) code element provides maximum amount ofseparation or delay between the first wavelength. For code sequence of255 individual elements, starting the second wavelength at the 127^(th)or 128^(th) code element provides maximum amount of separation or delaybetween the first wavelength. For code sequence of 511 individualelements, starting the second wavelength at the 255^(th) or 256^(th)code element provides maximum amount of separation or delay between thefirst wavelength. For code sequence of 1023 individual elements,starting the second wavelength at the 511^(st) or 512^(nd) code elementprovides maximum amount of separation or delay between the firstwavelength. For code sequence of 2047 individual elements, starting thesecond wavelength at the 1023^(rd) or 1024^(th) code element providesmaximum amount of separation or delay between the first wavelength. Forcode sequence of 4095 individual elements, starting the secondwavelength at the 2047^(th) or 2048^(th) code element provides maximumamount of separation or delay between the first wavelength. For codesequence of 8191 individual elements, starting the second wavelength atthe 4095^(th) or 4096^(th) code element provides maximum amount ofseparation or delay between the first wavelength. Alternatively, foreach of the code sequences of 31, 63, 127, 255, 511, 1023, 2047, 4095and 8191 individual elements described, the second wavelength can startat the code element representing 25, 26, 27, 28, 29, 30, 31, 32, 33, 34,35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48 or 49 percent ofthe total number of individual elements in the code sequence or anypercentage in between such percentages or any range of percentages inbetween 25 percent and 49 percent. Alternatively, for each of the codesequences of 31, 63, 127, 255, 511, 1023, 2047, 4095 and 8191 individualelements described, the second wavelength can start at the code elementrepresenting 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65,66, 67, 68, 69, 70, 71, 72, 73, 74 or 75 percent of the total number ofindividual elements in the code sequence or any percentage in betweensuch percentages or any range of percentages in between 51 percent and75 percent.

If three wavelengths are used with a code sequence of 31 individualelements and individual code element length of 1 nanosecond, the firstwavelength could be transmitted starting with the first code element,the second wavelength could be transmitted starting with the 10^(th) or11^(th) code element and the third wavelength could be transmittedstarting with the 20^(th) or 21^(st) code element. The amount ofseparation or delay between the first wavelength and second wavelengthin this example would be 9 nanoseconds and 10 nanoseconds fortransmission starting with 10^(th) and 11^(th) code element,respectively. Starting the second wavelength at the 10^(th) or 11^(th)code element and the third wavelength at the 20^(th) or 21^(st) codeelement provides maximum amount of separation or delay betweenwavelengths for code sequence of 31 individual elements. For codesequence of 63 individual elements, starting the second wavelength atthe 21^(st) code element and the third wavelength at the 42^(nd) codeelement provides maximum amount of separation or delay betweenwavelengths. For code sequence of 127 individual elements, starting thesecond wavelength at the 42^(nd) or 43^(rd) code element and the thirdwavelength at the 84^(th) or 85^(th) code element provides maximumamount of separation or delay between wavelengths. For code sequence of255 individual elements, starting the second wavelength at the 85^(th)code element and the third wavelength at the 170^(th) code elementprovides maximum amount of separation or delay between wavelengths. Forcode sequence of 511 individual elements, starting the second wavelengthat the 170^(th) or 171^(st) code element and the third wavelength at the340^(th) or 341^(st) code element provides maximum amount of separationor delay between wavelengths. For code sequence of 1023 individualelements, starting the second wavelength at the 341^(st) code elementand the third wavelength at the 682^(nd) code element provides maximumamount of separation or delay between wavelengths. For code sequence of2047 individual elements, starting the second wavelength at the 682^(nd)or 683^(rd) code element and the third wavelength at the 1364^(th) or1365^(th) code element provides maximum amount of separation or delaybetween the first wavelength. For code sequence of 4095 individualelements, starting the second wavelength at the 1365^(th) and the thirdwavelength at the 2730^(th) code element provides maximum amount ofseparation or delay between wavelengths. For code sequence of 8191individual elements, starting the second wavelength at the 2730^(th) or2731^(st) code element and the third wavelength at the 5460^(th) or5461^(st) code element provides maximum amount of separation or delaybetween wavelengths. Alternatively, for each of the code sequences of31, 63, 127, 255, 511, 1023, 2047, 4095 and 8191 individual elementsdescribed, the second wavelength can start at the code elementrepresenting 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31,32 or 33 percent of the total number of individual elements in the codesequence or any percentage in between such percentages or any range ofpercentages in between 17 percent and 33 percent. Alternatively, foreach of the code sequences of 31, 63, 127, 255, 511, 1023, 2047, 4095and 8191 individual elements described, the second wavelength can startat the code element representing 34, 35, 36, 37, 38, 39, 40, 41, 42, 43,44, 45, 46, 47, 48, 49 or 50 percent of the total number of individualelements in the code sequence or any percentage in between suchpercentages or any range of percentages in between 34 percent and 50percent. Alternatively, for each of the code sequences of 31, 63, 127,255, 511, 1023, 2047, 4095 and 8191 individual elements described, thethird wavelength can start at the code element representing 51, 52, 53,54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65 or 66 percent of thetotal number of individual elements in the code sequence or anypercentage in between such percentages or any range of percentages inbetween 51 percent and 66 percent. Alternatively, for each of the codesequences of 31, 63, 127, 255, 511, 1023, 2047, 4095 and 8191 individualelements described, the third wavelength can start at the code elementrepresenting 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81,82 or 83 percent of the total number of individual elements in the codesequence or any percentage in between such percentages or any range ofpercentages in between 67 percent and 83 percent.

FIG. 10 is a diagram illustrating the use of a 31-bit unipolar Galoiscode in one embodiment of the present invention. The chosen codesequence of digital modulation signal 16 is1100110100100001010111011000111 and starts with the third element or0011010010000101011101100011111. The code sequence has 31 individualcode elements and is unipolar. It is also a Galois code sequence. FIG.10 also illustrates the resulting digital modulation signal 16 afterusing the chosen 0011010010000101011101100011111 sequence andillustrates a code element length of 1 nanosecond. The chosen0011010010000101011101100011111 sequence is used on the first opticalillumination source at the first wavelength and the second opticalillumination source at the second wavelength in a circular or repeatingmanner i.e. multiple code sequences are used. The chosen0011010010000101011101100011111 sequence is used in a circular orrepeating manner with the first optical illumination source at the firstwavelength starting with the third element. After 13 nanoseconds, thechosen 0011010010000101011101100011111 sequence is used in a circular orrepeating manner with the second optical illumination source at thesecond wavelength. Alternatively, both optical illumination sources canbe started at the same time. A chosen 0011010010000101011101100011111sequence is used in a circular or repeating manner with the firstoptical illumination source at the first wavelength. A second version ofthe same chosen sequence beginning at the sixteen element is used in acircular or repeating manner with the second optical illumination sourceat the second wavelength. The second version of the same chosen sequenceis 1010111011000111110011010010000. FIG. 10 also illustrates theresulting temporal transfer characteristic or the temporal point spreadfunction of this example. A delay exists between the peaks of thetemporal transfer characteristic or the temporal point spread functionof the first wavelength and second wavelength.

Under another embodiment, Temporal Response Analysis Engine 11 analyzesand processes the correlation of detected response signal 19 with theelectronic reference signal 17 or source reference signal 18. Thecorrelation contains both the instrument response function and thetemporal transfer characteristic or the temporal point spread function.Temporal Response Analysis Engine 11 derives or separates the instrumentresponse function from the temporal transfer characteristic or thetemporal point spread function in the correlation. The temporal transfercharacteristic or the temporal point spread function can change overtime based on changes in properties or characteristics of sample 5 overtime, particularly if sample 5 is live human tissue. However, theinstrument response function can be less susceptible to change over timedepending on the stability of the hardware or equipment. TemporalResponse Analysis Engine 11 can measure the instrument response functionindependently without the temporal transfer characteristic or thetemporal point spread function by implementing a calibration procedurewhere the instrument response function is measured while sample 5 isremoved from the optical path between optical illumination source 3 andoptical detector 7. This removal can be accomplished by physicallyremoving sample 5 or altering the optical transmission path betweensample 5 and optical detector 7 to avoid sample 5. Once the instrumentresponse function is determined, the temporal transfer characteristic orthe temporal point spread function can be derived from the correlationof detected response signal 19 with the electronic reference signal 17or source reference signal 18.

Alternatively, the instrument response function can be approximatedwithout independent or direct measurement. By avoiding independent ordirect measurement, the calibration procedure is avoided. In addition,when photon measurement system 100 is operating for a longer period oftime, re-calibration may be required if photon measurement system 100drifts. By avoiding independent or direct measurement, re-calibration isalso avoided. The instrument response function is assumed to be fixed orconstant over time or varying slowly over time. Temporal ResponseAnalysis Engine 11 first obtains or stores a set of correlations ofdetected response signal 19 with the electronic reference signal 17 orsource reference signal 18. Each correlation can result from a singlecode sequence, multiple code sequences or multiple identical codesequences. Each correlation is associated with a given point in time.The set of correlations can comprise 20 correlations or 4, 5, 6, 7, 8,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 25, 50, 75, 100, 150, 200,250, 300, 350, 400, 450, 500 or any number in between such numbers orany range of correlations in between 4 and 500. Temporal ResponseAnalysis Engine 11 initially selects one of the correlations from theset of correlations which can be the first correlation. The selectedcorrelation can initially be assumed to be or treated as the instrumentresponse function.

Temporal Response Analysis Engine 11 generates a set of temporaltransfer characteristics or the temporal point spread functionsassociated with a range of photon times-of-flight, fluorescencelifetimes, tissue absorption coefficients, tissue scatteringcoefficients, location of fluorescing material or other tissueproperties or characteristics, either prior to or after selection of theselected correlation. For noninvasive human cerebral oximetry, TemporalResponse Analysis Engine 11 generates a set of temporal transfercharacteristics or the temporal point spread functions associated with arange of oxygenated and deoxygenated hemoglobin levels.

Temporal Response Analysis Engine 11 convolves the set of temporaltransfer characteristics or the temporal point spread functions with theselected correlation resulting in a set of convolutions. The set ofconvolutions is compared with the set of correlations using the leastsquared method and the difference recorded or stored. Temporal ResponseAnalysis Engine 11 then modifies the selected correlation or assumedinstrument response function and convolves the set of temporal transfercharacteristics or the temporal point spread functions with the modifiedcorrelation resulting in a set of modified convolutions. The set ofmodified convolutions is compared with the set of correlations using theleast squared method and the difference recorded or stored. TemporalResponse Analysis Engine 11 repeats the steps or process for differentmodified or assumed instrument response functions iteratively until thedifference between the set of modified convolution and the set ofcorrelations is minimized under the least squared function analysis. Themodified convolution or assumed instrument response function thatresults in the minimal difference between the set of convolutions andthe set of correlations is treated as or assumed to be the actualinstrument response function. The temporal transfer characteristic orthe temporal point spread function is separated from the instrumentresponse function and then used as described to obtain μ_(a) and theconcentrations of interest.

FIG. 11 is a diagram illustrating the placement of source and detectorto measure total hemoglobin under one embodiment of the presentinvention. Under this embodiment, photon measurement system 100 is usedto measure total hemoglobin level in human tissue. Total hemoglobin isan important parameter that can be used to guide many clinicalinterventions. It is the sum of the concentrations of oxyhemoglobin,reduced (or deoxy-) hemoglobin, carboxyhemoglobin, and methemoglobin Anapproximation can be utilized to use only oxygemoglobin and reducedhemoglobin to calculate total hemoglobin; this approximation is robustin healthy individuals.

Total hemoglobin can be also essential to the accurate calculation ofblood oxygen saturation. Fractional oxygen saturation can be determinedby dividing the concentration of oxyhemoglobin by the concentration oftotal hemoglobin. If only oxyhemoglobin plus reduced hemoglobin is usedto calculate total hemoglobin, the resulting oxygen saturation can bedefined as functional oxygen saturation.

Accurate measurement of total hemoglobin can be vital, yet the majorityof point-of-care hemometry devices actually measure hematocrit and applyan assumption of hemoglobin content per cell to calculate hemoglobin.Hematocrit is the percentage of red blood cells in whole blood. Toconvert from hematocrit to hemoglobin, the mean corpuscular hemoglobinconcentration must be known. This value ranges from 32 to 36 g/dL innormal individuals.

Hematocrit can be measured in many blood gas analyzers by conductimetry.This technique is based on the principle that plasma is rich inelectrolytes and is highly conductive, whereas blood cells arenon-conductive. Thus, the greater the electrical conduction, the fewerthe cells. While this technique can be fairly robust under normalphysiological conditions, many illnesses and therapies can causeerroneous readings. Fluid resuscitation, for example, can producesignificant hemodilution with hyperosmolar solutions. The injection ofradiographic contrast media, which has very high osmolarity, can greatlyincrease conduction while not significantly diluting the blood. Someprotocols can include the administration of proteins, such as albumin,which are similar to blood cells in that they are non-conductive. As aresult, conductimetric readings can be very inaccurate and can adverselyimpact the course of treatment for a patient.

Other point-of-care devices can extract a small volume of blood from afingerprick to measure hematocrit. However, the process of applyingpressure to the capillary bed can result in a different proportion ofplasma to extracted red blood cells as exists in the vessels.

One spectrophotometry system has the potential to accurately measurehemoglobin itself without need for assumptions or calculations and isnot adversely affected by hemodilution or hyperosmolar solutions. Thechallenges of using such spectrophotometry to measure hemoglobin includecontributions from other skin chromophores, variations in blood vessellocation and density, and changes in vessel diameter and subsequentoptical path length during pulsation. Measurement of arterial diameteror pulsatile path length is a prerequisite for accurate noninvasivedetermination of hemoglobin concentration with such spectrophotometry.

The total hemoglobin system of one embodiment of the present inventionmitigates the problems encountered by other methods and provides anaccurate measure of hemoglobin concentration, with inherent opticalpathlength correction. Modulated light at two to four or morewavelengths using a pseudo-random sequence is detected either intransmission mode, such as through a finger or earlobe, or in reflectionmode, such as through the forearm, temple, neck, or other suitablelocation. The detected light contains the temporal response of thesample, which may be fit using diffusion theory or similar techniques toobtain the absorption coefficient, the scattering coefficient, and theoptical path length independently. The absorption coefficient, μ_(a), isrelated to concentration by μ_(a)=2.303(ε₁C₁+ε₂C₂+ . . . ε_(n)C_(n))where n is the number of absorbing species present in the sample, ε isthe wavelength-dependent molar absorption coefficient (a knownconstant), and C is the concentration. Hemoglobin is the primaryabsorbing species in blood and tissue. To determine the concentrationsof oxyhemoglobin and reduced hemoglobin, measurements at two wavelengthscan be used to isolate each contribution. Alternatively, a singlemeasurement at the isosbestic point for the two species, 805 nmwavelength, can give the combined concentration, i.e. total hemoglobinunder the approximation that oxy- and reduced hemoglobin dominate. For amore accurate measurement, additional measurements at additionalwavelengths can be included to account for other absorbing species. Itis not necessary to add wavelengths to account for scatter. To determinethe concentrations of all four of the primary forms of hemoglobin,oxyhemoglobin, reduced hemoglobin, carboxyhemoglobin, and methemoglobin,measurements at a minimum of four wavelengths can be used to isolateeach contribution.

Wavelengths used can be in the range of 650 nm to 1000 nm preferably 850nm, 805 nm (isosbestic point of oxy-and reduced hemoglobin), 780 nm(isosbestic point of reduced hemoglobin and methemoglobin), 760 nm, 660nm (isosbestic point of reduced hemoglobin and methemoglobin), and 630nm. Additional wavelengths can be added to compensate for waterabsorption and, if desired, bilirubin absorption. The preferredwavelengths for bilirubin absorption is 400-500 nm, more specifically450 nm.

The absorption coefficient obtained by transmission or reflectionmeasurement through at least in part a blood vessel, can also includecontributions from surrounding tissue, capillary beds, and venous blood.To obtain the absorption coefficient due solely to arterial hemoglobin,measurements can be acquired at systole and diastole and the resultingabsorption coefficients subtracted from one another to remove unwantedcontributions. Using multiple wavelengths as described, hemoglobinconcentration can be determined from absorption coefficients. It isdesirable to measure at least twice during systole and twice duringdiastole or 3, 4 or 5 times during systole and diastole, preferably 6,7, 8, 9 or 10 times during systole and diastole. Additionally, a triggercould be utilized to trigger measurement during the lowest pressure ofdiastole and highest pressure of systole. Photon measurement system 100can be used to take measurements during systole and diastole with heartrates of 60 to 100 beats per minute and up to 220 beats per minute. Witha heart rate of 60 beats per minute and two measurements for systole anddiastole, photon measurement system 100 would take a minimum of fourmeasurements per second or measure at a rate of 0.25 seconds permeasurement. The code sequence utilized would not exceed 0.25 seconds.With a heart rate of 220 beats per minute and 10 measurements forsystole and diastole, photon measurement system 100 would take a minimumof 73⅓ measurements per second or measure at a rate of 0.0136 secondsper measurement. The code sequence utilized would not exceed 0.0136seconds. Photon measurement system 100 can take measurements at a rateof 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50, 60, 70 or 73⅓ measurementsper second or any rate in between such rates or any range of rate inbetween 4 and 73⅓ measurements per second. Photon measurement system 100can measure at a rate of 0.0136, 0.015, 0.0175, 0.02, 0.03, 0.04, 0.05,0.075, 0.1, 0.125, 0.15, 0.2, 0.25 seconds per measurement or any ratein between such rates or any range of rate in between 0.0136 and 0.25seconds per measurement. Prior systems utilizing short-pulsed lasers andsingle photon counting detectors are not sufficiently fast to makeenough measurements during the heartbeat cycle. On the other hand,photon measurement system 100 can derive the temporal transfercharacteristic or the temporal point spread function and determineabsorption coefficients or hemoglobin levels with a single code sequenceproviding the capability to make measurements at the rates described.

Alternatively, the source or plurality of sources and the detector orplurality of detectors can be aligned over a blood vessel or capillarybed such as an artery near the temple with the source or plurality ofsources and the detector or plurality of detectors spaced apartapproximately twice the distance that the artery is deep. This alignmentensures the photon path travels at least in part through the artery.Depth discrimination can be possible utilizing time-domain information.Photons that have traveled longer paths have longer arrival times.

When measuring vessels near the surface, external temperature controlcan be necessary as local blood flow may vary with temperature.Temperature control can be achieved by placing one or more heaters inthe unit contacting the patient and a temperature feedback system in themain instrument.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and many modifications andvariations are possible in light of the above teaching. The embodimentswere chosen and described in order to best explain the principles of theinvention and its practical application, to thereby enable othersskilled in the art to best utilize the invention and various embodimentswith various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto and their equivalents.

What is claimed is:
 1. A total hemoglobin measurement system formeasuring total hemoglobin in human tissue comprising: a signalgenerator configured to generate a digital modulation signalrepresenting a code sequence; an optical illumination source coupled tothe signal generator configured to receive the digital modulation signaland transmit a modulated optical signal along an optical transmissionpath to the human tissue in response to the digital modulation signal; adetector optically coupled to the human tissue configured to receive themodulated optical signal after transmission through the opticaltransmission path through the human tissue; and a processor coupled tothe detector configured to determine a temporal transfer characteristicfor the optical signal received by the detector and determine the totalhemoglobin based on at least the temporal transfer characteristic. 2.The total hemoglobin measurement system of claim 1 further comprising: adelay component optically coupled to the optical illumination sourceconfigured to increase a length of the optical transmission path.
 3. Thetotal hemoglobin measurement system of claim 2 wherein the delaycomponent comprises a reflective surface.
 4. The total hemoglobinmeasurement system of claim 1 wherein the code sequence comprisesmultiple identical code sequences of a single code sequence.
 5. Thetotal hemoglobin measurement system of claim 1 wherein the code sequenceis circular.
 6. The total hemoglobin measurement system of claim 1wherein the code sequence is orthogonal.
 7. The total hemoglobinmeasurement system of claim 1 wherein the code sequence is unipolar. 8.The total hemoglobin measurement system of claim 1 wherein the codesequence is a Galois code.
 9. The total hemoglobin measurement system ofclaim 1 wherein the code sequence is a maximal-length sequence.
 10. Thetotal hemoglobin measurement system of claim 1 further comprising: asecond signal generator configured to generate a second digitalmodulation signal representing a second code sequence; a second opticalillumination source coupled to the second signal generator configured toreceive the second digital modulation signal and transmit a secondmodulated optical signal to the human tissue in response to the seconddigital modulation signal.
 11. The total hemoglobin measurement systemof claim 10 wherein the second code sequence and the code sequence havethe same ordering of code elements and begin at different code elements.12. A method for measuring total hemoglobin in a human tissuecomprising: generating a digital modulation signal associated with acode sequence; generating a modulated optical signal based at least onthe digital modulation signal; transmitting the modulated optical signalto the human tissue; receiving a modified version of the modulatedoptical signal after transmission through the human tissue; determininga temporal transfer characteristic for the modified version of themodulated optical signal; and determining the total hemoglobin based onat least the temporal transfer characteristic.
 13. The method of claim12 further comprising: delaying the modulated optical signal to reducenoise effects.
 14. The method of claim 12 further comprising:determining an absorption coefficient of the human tissue from thetemporal transfer characteristic.
 15. The method of claim 12 wherein thecode sequence is circular.
 16. The method of claim 12 furthercomprising: determining a first absorption coefficient during systolebased on at least the temporal transfer characteristic; and determininga second absorption coefficient during diastole based on at least thetemporal transfer characteristic.
 17. The method of claim 12 wherein thecode sequence is orthogonal.
 18. The method of claim 12 furthercomprising: generating a second digital modulation signal associatedwith a second code sequence; generating a second modulated opticalsignal based on at least the second digital modulation signal;transmitting the second modulated optical signal to the human tissue;receiving a second modified version of the second modulated opticalsignal after transmission through the human tissue; and determining asecond temporal transfer characteristic for the second modified versionof the second modulated optical signal.
 19. The method of claim 12further comprising: delaying the code sequence to generate a second codesequence; generating a second digital modulation signal associated withthe second code sequence; generating a second modulated optical signalbased on at least the second digital modulation signal; transmitting thesecond modulated optical signal to the human tissue; receiving a secondmodified version of the second modulated optical signal aftertransmission through the human tissue; and determining a second temporaltransfer characteristic for the second modified version of the secondmodulated optical signal.
 20. A total hemoglobin measurement system formeasuring total hemoglobin in human tissue comprising: means forgenerating a digital modulation signal associated with a code sequence;means for generating a modulated optical signal based on at least thedigital modulation signal; means for transmitting the modulated opticalsignal to the human tissue; means for receiving a modified version ofthe modulated optical signal after transmission through the humantissue; means for determining a temporal transfer characteristic for themodified version of the modulated optical signal; and means fordetermining the total hemoglobin based on at least the temporal transfercharacteristic.